its a '59 plexi according to guitarbuyer magazine, they did a 'tone-zone' article with him. The plexi won't get very distorted unless pushed hard or used with a powersoak itll just have a lot of 'girth', and i doubt that it would be as prominent in the mix as the super reverb either, but the 4x12 cab and the el34s would boost the midrange/bass response that the open-backed super reverb would lack, especially since it uses 10 inch speakers and marr is apparently exclusively using vintage fender jaguars, a combination that wouldn't make for the fullest sound.
Saying all this, I doubt he'd bother with it in the studio. Or he could do what lee ranaldo (sonic youth) did to his super reverb, chop it up and make it into a head/closed back 4x10 configuration; would get a lot of the midrange and bass response back.
hope that helps!